In highly dispersed pseudo-linear high bit-rate long-haul transmission systems, intra-channel four-wave mixing (IFWM) and intra-channel cross-phase modulation (IXPM) are the main causes for nonlinear penalties. Using a Differential-phase-shift-keying (DPSK) modulation format can mitigate these penalties as compared to on-off-keying (OOK). Several 40-Gb/s DPSK ultra-long-haul DWDM transmission experiments have shown excellent capacity and distance performance. The strong resistance of DPSK against IXPM stems from the equal pulse energy in each time slot. It has also been shown that the reduced pulse energy of Return-to-Zero (RZ)-DPSK compared to RZ-OOK and a correlation between the nonlinear phase shifts of two adjacent pulses contribute to the better robustness of RZ-DPSK against IFWM.
In principle, alternating the polarization of the signal from bit to bit will further reduce the nonlinear penalties since there is little FWM between two orthogonally polarized signals and the effect of XPM between the two orthogonal polarizations is half that between the parallel ones. Prior art simulations have shown that more than 50% improvement in the transmission distance can be achieved by using Alternate Polarization-DPSK (APol-DPSK) compared to single polarization DPSK. The simultaneous application of DPSK and polarization alternation enhances the performance in an optical transport system where the intra-channel optical nonlinear interactions are limiting factors.
One prior art method for implementing alternate-polarization DPSK uses optical-time-division-multiplexing (OTDM) and polarization-multiplexing in the transmitter. The method requires a short-pulse source operating at half bit rate, OTDM demultiplexing or polarization demultiplexing at receiver, and complicated transmitters and receivers. Such a method is not practical in real system applications and is not suitable for Non-Return-to-Zero (NRZ) formats. This approaches is also impractical for optical networks because it requires multiple optical transmitters for a single channel.
Moreover, prior art methods of DPSK data precoding and encoding are not suitable for efficiently generating alternate polarization formatted signals.